Shigellosis: Causes and Symptoms

Shigella share several of the characteristics of a group of bacteria that inhabit the intestinal tract. E. coli, another cause of food-borne illness, can be mistaken for Shigella both by physicians and the laboratory. Careful testing is needed to assure proper diagnosis and treatment.

Shigella are very resistant to the acid produced by the stomach, and this allows them to easily pass through the gastrointestinal tract and infect the colon (large intestine). The result is a colitis that produces multiple ulcers, which can bleed. Shigella also produce a number of toxins (Shiga toxin and others) that increase the amount of fluid secretion by the intestinal tract. This fluid secretion is a major cause of the diarrhea symptoms.

Shigella infection spreads through food or water contaminated by human waste. Sources of transmission are:

• contaminated milk, ice cream, vegetables and other foods which often cause epidemics
• household contacts (40% of adults and 20% of children will develop infection from such a source)
• poor hygiene and overcrowded living conditions
• day care centers
• sexual practices which lead to oral-anal contact, directly or indirectly

Symptoms can be limited to only mild diarrhea or go on to full-blown dysentery. Dehydration results from the large fluid losses due to diarrhea, vomiting and fever. Inability to eat or drink worsens the situation.

In developed countries, most infections are of the less severe type, and are often due to S. sonnei. The period between infection and symptoms (incubation period) varies from one to seven days. Shigellosis can last from a few days to several weeks, with an average of seven days.

Areas outside the intestine can be involved, including:

• nervous system (irritation of the meninges or meningitis, encephalitis, and seizures)
• kidneys (producing hemolytic uremic syndrome or HUS which leads to kidney failure)
• joints (leading to an unusual form of arthritis called Reiter's syndrome)
• skin (rash)

One of the most serious complications of this disease is HUS, which involves the kidney. The main findings are kidney failure and damage to red blood cells. As many as 15% of patients die from this complication, and half the survivors develop chronic kidney failure, requiring dialysis.

Another life-threatening condition is toxic mega-colon. Severe inflammation causes the colon to dilate or stretch, and the thin colon wall may eventually tear. Certain medications (particularly those that diminish intestinal contractions) may increase this risk, but this interaction is unclear. Clues to this diagnosis include sudden decrease in diarrhea, swelling of the abdomen, and worsening abdominal pain.

— David Kaminstein, MD